Combustion of alkaline cooking liquor

ABSTRACT

The present invention relates to a method of treating evaporated cooking liquor obtained in pulp production on an alkali basis in order to recover alkali carbonate and sulphide. Black liquor is injected axially into a cylindrical furnace where it is dried and pyrolized by contact with a tangentially supplied oxygen-containing gas and then sulphidized to form alkali carbonate and alkali sulphide while in contact with a tangentially supplied inert gas. Gases formed during the process are subsequently contacted with another tangentially supplied oxygen-containing gas for oxidation thereof.

United States Patent 119.1

Holme Feb. 18, 1975 1 COMBUSTION 0F ALKALINE COOKING 2,9l4,386 11/1959 Shnpleigh 1. 23/277 R LIQUOR 3,333,917 8/1967 3,366,535 H1968 Inventor: Gunner Holme, D1ursh0lm, 3,574,051 4/1971 Shah 102/30 Sweden [73] Assignee: Angpanneforeningen, Stockholm, Primary Examiners' Leon Bashore Sweden Assistant Examiner-Alfred DAndrea Attorney, Agent, or FirmFred C. Philpitt [22] Filed: Mar. 26, 1973 [21] Appl. No.: 344,652 [57] ABSTRACT The present invention relates to a method of treating [30] Foreign Application Priority Data evaporated cooking liquor obtained in pulp produc- Apr. 4, 1972 Sweden 4304/72 on an alkali basis in Order recover alkali bonate and sulphide. [52] US. Cl. 162/30, 423/207 Black liquor is injected axially into a cylindrical [51] Int. Cl. D2lc 11/12 f ac where it is dried and pyrolized by contact with [58] Field Of Search l62/30-36; a tangentially supplied oxygen-containing gas and then 423/207; 23/277 R; 1l0/7 7 S sulphidized to form alkali carbonate and alkali sulphide while in contact with a tangentially supplied [56]. References Cited inert gas. Gases formed during the process are UNITED STATES PATENTS subsequently contacted with another tangentially 2.078.015 5/1954 Soderlund m .11. 110/7 B Supplied oxygen'comaining gas Oxidation thereof 2,808,011 10/1957 Miller ct a1 110/7 B 2911,2224 11/1959 Hochmuth 423/207 12 Chums 3 D'awmg F'gures Dry 9 Melting Pyrolysis Sulphidizing Finaloxidulion a, M, a A (melted) ;N g

N02 605- Hz Gas-H "'0 H 1i; PPr. 700c tconv piizs 200C 2 Steam Oil I//// lll 111111 1 Air Gz rs Inert gas Flue gas) 2504 ME Mttag dbe c/rcu N0 C0 +Nc1 S COMBUSTION OF ALKALINE COOKING LIQUOR BACKGROUND The soda process was the first alkaline regenerating process, in which combustion was combined with the recovery of chemicals. Herein alkali was recovered in the form of sodium carbonate. Later the process was modified and in the sulphate process now commonly used sulphide is recovered in addition to carbonate. The losses of chemicals are here replaced by sodium sulphate. The regenerating assembly, however, has maintained its original name ofsoda recovery boiler, which is misleading. Since, however, other designations could cause misunderstandings, the designation recovery boiler is used herein as previously for the heat and chemical recovery assemblies of the sulphate pulp mills and the sulphite pulp mills on sodium basis.

The recovery boiler currently used is based on the designs worked out by Tomlinson in the 1930s and it then replaced the rotary unit plus the melting furnace with water cooled air ports used up to that time. These assemblies had a capacity corresponding to pulp prothe method of air supply, cleaning equipment, the tube arrangement of the boiler economizer design etc. Above all it is the increase of capacity, which is strikingly large. At present the units have a capacity of 1,000 tons pulp per 24 hours and the problems of the recovery boiler, which were observable already several years ago, when the designs were extrapolated to ever larger units, have not become considerable. In a mod ern recovery boiler the liquor is sprayed into the furnace in relatively coarse drops, which on one hand temporarily adhere to the walls in the furnace and on the the other hand fall down to the bottom thereof. Combustion air is supplied in air registers, which are located at varying height above the bottom. It is desired to obtain such an atmosphere in the lower part of the furnace that a maximum quantity of carbonates and sulphides is formed from the sodium salts. Further upwards in the furnace it is desired to obtain a combustion as complete as possible of all combustible compo: nents, including certain odorous components such as e.g. hydrogen sulphide. At the same time it is highly desirable that a minimum quantity of alkali salts should be carried over from the combustion zone up into the super heater of the boiler and other heat surfaces. Since the assemblies have now attained such large dimensions theree are difficulties in controlling the reduction and oxidation processes by means of the air streams from the registers. The consequence is that the emission of odorous gases is not so easily prevented. Since the combustion process is highly nonhomogeneous and the areas are so large, it is to be feared that difficulties will arise in attempts at restricting the emis:

in the furnace of the recovery boiler means a considerable risk of a destructive explosion, which is at present clearly understood within the sulphate industry and the causes of which it is highly desirable to remove. One method of reducing the risk is to maintain a low temperature of the tube material. If the steam pressure in the boiler is low, this requirement is met. However, this feature has as a consequence that the possibility of generating back pressure power is reduced, which may have a large economic importance. At present a recovery boiler will be very large relative the amount of heat which is converted, as compared to an oil heated unit. The furnace volume, e.g., is about 6 times as large. Consequently, the recovery boiler becomes expensive.

The drawback of the Tomlinson unit at present is the obvious heterogeneity of the reduction and oxidation zones. It is therefore necessary to form a reaction chamber with a large turbulence and a high heat load. Such a device is available in the cyclone burner (known per se).

THE PRESENT INVENTION The method according to the present invention is characterized in that the liquor is injected into one end of a cylindrically shaped reaction chamber with substantially tangential gas supply, and the reaction are varied so that in a first zone, as seen from the injection end, the liquor is dried and pyrolyzed, in a second zone is melted and sulphidized, and in at least one succeeding additional zone is oxidized, the melt containing al- .kali carbonate and alkali sulfide being tapped in connection with the second zone.

The invention will now be described more closely with reference to the attached schematical drawings, in which FIG. 1 illustrates a section through a cylindrically shaped reaction chamber with a substantially tangential gas supply, FIG. 2 in cross section and at a reduced scale shows a plant comprising such a reaction chamber connected to a steam boiler, and FIGS schematically illustrates the reactions occurring in a reaction chamber according to FIG. 1.

The exemplary primary furnace in a plant for performing the method according to the present invention comprises an horizontal cylinder 2v with a tangential air supply along the mantle, indicated by means of arrows 4. The cylinder is connected with a steam boiler 6, having a form similar to a so called slag top furnace boiler for mineral coal and brown coal. The end of the cylinder remote from the boiler is closed by means of an end wall 8, at which the liquor and the fuel oil required, if any, is added at 10 and 12, respectively. The cylinder 2, the cyclone, is provided with tapping means for the smelt indicated at 14. The cylinder is provided with a radial flange 16. If desired a plurality of such radial flanges may divide the cylinder into two or more zones.

The internal mantle surface is conveniently cooled in a manner not illustrated. This cooling can be performed in various ways without the risk of smelt soda explosions occurring. In principle it is possible, e.g., to cool by means of saturated steam from the drum. Due to the superheating occuring thereby and a low heat transfer value the material temperature rises and may cause an increased corrosion risk, it is true, but this risk can be counteracted, e.g. by selecting compound tubes with austenitic material externally as cooling tubes.

In a combustion cyclone such as the one described above it is possible in each zone at each moment to provide exactly the conditions required so as to attain a desired result.

The pyrolyzis process, which forms part of the intended total process in the cyclone furnace, can be caused to proceed very rapidly (shock pyrolysis). For the practical adaption it is necessary that the cyclone combustion does not produce a larger but preferably considerably smaller quantity of transferred alkali salts to the tube system in the boiler.

A high temperature of the combustion air is necessary if it is desired to avoid, to the greatest possible extent, oil addition into the process.

This is also favourable from the point of view that it will then be enough room in the heat balance ofthe boiler for an increased preheating of the feed water to the boiler with extraction steam from the back pressure turbine, which like the steam pressure increase provides an improved power yield.

In FIG. 3 the reactions occurring are illustrated more in detail.

As mentioned above the present invention relates to a method of treating evaporated cooking liquor obtained in pulp production on alkali basis. The object has been to eliminate the disadvantages mentioned above of a so called recovery boiler. As mentioned it is desirable in such a boiler to obtain a maximum quantity of carbonate and sulphide. Also in the method according to the present invention the object is the same. As mentioned further, the reduction and oxidation zones are very heterogeneous in the present recovery boiler, with the accompanying disadvantages described. The heterogeneity within the reduction zone means that drying-pyrolyzis and melting-sulphidizing does not occur in zones well defined from each other, whereby it is very difficult to control the process. Neither are the reduction and oxidation zones so defined from each other as might be desired.

All the disadvantages mentioned above are eliminated by replacing the prior boiler by a so called cyclone. This provides the possibility of controlling the reaction process at each moment in each reaction zone, as mentioned above.

In FIG. 3 the above-mentioned conditions are illustrated. It should be observed that the reaction components and the reactions indicated are known per se in the prior recovery boiler. The difference is that by means of the new method the various reactions can be controlled in a manner, which can provide the desired result in each step.

The liquor arriving into the cyclone at the left hand side of FIG. 3 is finally atomized and enters with hot flue gases. Hereby a drying and pyrolysis of the liquor occurs, whereby pure Na- CO is formed and therein pure coal. In addition combustion gas is formed, which contains all the sulphur in the form of H 8. The components in said gas are illustrated on the drawing. The coal continues to burn, whereby the temperature rises, so that Na CO smelts in the melting and sulphidizing zone, and the coal burns to an end. The H 5 in the combustion gas partly creeps into the smelt of Na CO and a reaction occurs according to the following formula:

N21 CO H S N3 5 l CO2 H2O Consequently, Na s is formed, which is present in a molten state. In addition the tapped smelt contains Na S0 The combustion gas depleted of ms is transferred to final oxidation, wherein the result will be the gas shown at the right hand end of FIG. 3, said gas being fed into the furnace. In the final oxidation zone molten sodium sulphate may splash and is tapped in the manner indicated.

By replacing the prior reaction boiler by a cyclone a reaction chamber has been obtained with a large turbulence and high heat load, from which the many substantial advantages of the present invention can be derived. The temperatures required for the various reactions are well known per se and in the first zone are about 700C as illustrated in FIG. 3, in the second zone about 900C and in the final oxidation zone about 1 ,l00l,200C. In addition it has been mentioned that oil or gas can be supplied simultaneously with the cooking liquor and that the walls of the reaction chamber can be cooled by tubes. This feature together with the tangential air supply as mentioned above provides several possibilities of attaining the desired division into reaction zones. The division into the two first reaction zones may consequently be controlled partly by controlling the tangential air supply along the mantle, partly by controlling the cooling of the mantle and partly by controlling the ratio of liquor/oil. For additional limitation a low partition wall is possible between the two first zones. The reduction and oxidation zones are separated in a natural manner by the partition wall 16. It can also possibly be of advantage to divide the final oxidation step into one or more zones by means of similar partition walls. The reaction conditions can also be controlled by controlling the pressure in the various reaction zones. The pressure may thus be increased by reducing the passage area of the succeeding partition wall. Regarding the gas utilized for the tangential gas supply said gas is preferably air in the first zone, an inert gas, e.g. flue gas, in the second zone and again air in the final oxidation zone.

The advantages of the present invention may be summarized as follows:

the emission of odorous gases is controlled,

improved fuel economy,

no risk of smelt soda explosions,

substantially improved power yield in back pressure turbines,

smaller dimensions and possibly lower cost of installation,

fewer interruptions for inspection and repair.

What is claimed is:

l. A method of treating evaporated alkaline cooking liquor obtained from pulp production in order to recover alkali carbonate and alkali sulphide, comprising introducing a plurality of streams of hot oxygenvcontaining gas tangentially into a cylindrical reaction chamber to obtain a first area of tangential gas supply extending substantially from one end of said chamber in the axial direction thereof, injecting evaporated liquor of the aforesaid type into said end of the chamber and into contact with said tangential gas supply, controlling the reaction conditions within said first area to obtain a temperature level in a first zone sufficient to dry and pyrolize the injected liquor into alkali carbonate and carbon, and also causing the formation of combustion gas containing the sulphur in the injected liquor in the form of H S, controlling the reaction conditions in a second zone following said first zone in the axial direction of the cylindrical chamber to obtain in said second zone a temperature level sufficient to smelt said alkali carbonate and cause a sulphidizing reaction so that a smelt containing alkali carbonate and alkali sulphide is obtained which is removed by tapping from said second zone, introducing a plurality of streams of hot oxygen-containing gas tangentially into said cylindrical chamber to obtain a second area of tangential gas supply extending substantially from said second zone in the axial direction of said chamber, controlling the reaction conditions within said second area to obtain a temperature level in at least one additional zone therein sufficient to oxidize the combustion gases from the preceding zones.

2. The method according to claim 1, comprising introducing a plurality of streams of an inert gas tangentially into said chamber to obtain a third area of tangential gas supply between said first and second areas and containing said second zone.

3. The method according to claim 1, comprising supplying oil simultaneously with said evaporated liquor into said end of the chamber.

4. The method according to claim 1 comprising supplying gas simultaneously with said evaporated liquor into said end of the chamber.

5. The method according to claim 1 wherein the steps of controlling the reaction conditions within said zones include cooling the walls of the reaction chamber by means of tubes containing streaming steam.

6. The method according to claim 1 wherein the steps of controlling the reaction conditions within said zones include controlling the tangential gas supply.

7. The method according to claim 3, wherein the steps of controlling the reaction conditions within said zones comprise varying the ratio of injected liquor/injected oil.

8. The method according to claim 4 wherein the steps of controlling the reaction conditions within said zones comprise controlling the ratio of injected liquor/injected gas.

9. The method according to claim 1 wherein the steps of controlling the reaction conditions within said first and second zones include dividing the reaction chamher by means of an apertured partition wall between said first and second zones.

10. The method according to claim 1 wherein the steps of controlling the reaction conditions within said second and third zones include dividing the reaction chamber by means of an apertured partition wall between said second and third reaction zones.

11. The method according to claim 1, wherein the steps of controlling the reaction conditions within said zones include dividing the reaction chamber by means of at.least two apertured partition walls.

12. The method according to claim 11 wherein the passage area of at least one of said partition walls is controlled. 

1. A METHOD OF TREATING EVAPORATED ALKALINE COOKING LIQUOR OBTAINED FROM PULP PRODUCTION IN ORDER TO RECOVER ALKALI CARBONATE AND ALKALI SULPHIDE, COMPRISING INTRODUCING A PLURALITY OF STREAMS OF HOT OXYGEN-CONTAINING GAS TANGENTIALLY INTO A CYLINDRICAL REACTION CHAMBER TTO OBTAIN A FIRST AREA OF TANGENTIAL GAS SUPPLY EXTENDING SUBSTANTIALLY FROM ONE END OF SAID CHAMBER IN THE AXIAL DIRECTION THEREOF, INJECTING EVAPORATED LIQUOR OF THE AFORESAID TYPE INTO SAID END OF THE CHAMBER AND INTO CCONTACT WITH SAID TANGENTIAL GAS SUPPLY, CONTROLLING THE REACTION CONDITIONS WITHIN SAID FIRST AREA TO OBTAIN A TEMPERATURE LEVEL IN A FIRST ZONE SUFFICIENT TO DRY AND PYROLIZE THE INJECTED LIQUOR INTO ALKALI CARBONATE AND CARBON, AND ALSO CAUSING THE FORMATION OF COMBUSTION GAS CONTAINING THE SULPHURIN THE INJECTED LIQUOR IN THE FORM OF H2S, CONTROLLING THE REACTION CONDITIONS IN A SECOND ZONE FOLLOWING SAID FIRST ZONE IN THE AXIAL DIRECTION OF THE CYLINDRICAL CHAMBER TO OBTAIN IN SAID SECOND ZONE A TEMPERATURE LEVEL SUFFICIENT TO SMELT SAID ALKALI CARBONATE AND CAUSE A SULPHIDIZING REACTION SO THAT A SMELT CONTAINING ALKALI CARBONATE AND ALKALI SULPHID IS OBTAINED WHICH IS REMOVED BY TRAPPING FROM SAID SECOND ZONE INTRODUCING A PLURALITY STREAMS OF HOT OXYGEN-CONTAININGG GAS TANGENTIALLY INTO SAID CYLINDRICAL CHAMBER TO OBTAIN A SECOND AREA OF TANGENTIAL GAS SUPPLY EXTENDING SUBSTANTIALLY FROM SAID SECOND ZONE IN THE AXIAL DIRECTION OF SAID CHAMBER, CONTROLLING THE REACTION CONDITIONS WITHIN SAID SECOND AREA TO OBTAIN A TEMPERATURE LEVEL IN AT LEAST ONE ADDITONAL ZONE THEREIN SUFFICIENT TO OXIDIZE THE COMBUSTION GASES FROM THE PRECEDING ZONES.
 2. The method according to claim 1, comprising introducing a plurality of streams of an inert gas tangentially into said chamber to obtain a third area of tangential gas supply between said first and second areas and containing said second zone.
 3. The Method according to claim 1, comprising supplying oil simultaneously with said evaporated liquor into said end of the chamber.
 4. The method according to claim 1 comprising supplying gas simultaneously with said evaporated liquor into said end of the chamber.
 5. The method according to claim 1 wherein the steps of controlling the reaction conditions within said zones include cooling the walls of the reaction chamber by means of tubes containing streaming steam.
 6. The method according to claim 1 wherein the steps of controlling the reaction conditions within said zones include controlling the tangential gas supply.
 7. The method according to claim 3, wherein the steps of controlling the reaction conditions within said zones comprise varying the ratio of injected liquor/injected oil.
 8. The method according to claim 4 wherein the steps of controlling the reaction conditions within said zones comprise controlling the ratio of injected liquor/injected gas.
 9. The method according to claim 1 wherein the steps of controlling the reaction conditions within said first and second zones include dividing the reaction chamber by means of an apertured partition wall between said first and second zones.
 10. The method according to claim 1 wherein the steps of controlling the reaction conditions within said second and third zones include dividing the reaction chamber by means of an apertured partition wall between said second and third reaction zones.
 11. The method according to claim 1, wherein the steps of controlling the reaction conditions within said zones include dividing the reaction chamber by means of at least two apertured partition walls.
 12. The method according to claim 11 wherein the passage area of at least one of said partition walls is controlled. 